1. Field of the Invention
The present invention relates to methods of controlling link quality and transmit power used for transmissions between a base station and one or more mobile stations of a network such as a general packet radio service (GPRS) network or enhanced general packet radio service (EGPRS) network.
2. Description of Related Art
In cellular systems, a network of base transceiver stations (BTS) is used to provide wireless links with mobile stations. The mobile stations, sometimes referred to as mobiles, typically communicate via analog or digital modulated radio frequency (RF) signals with the base station, which is itself connected to an external network.
A variety of digital cellular networks and telecommunications standards are in use today, such as GSM (Global System for Mobile Communication), and GSM derivatives (e.g., DCS 1800, PCS 1900, etc.). GPRS (General Packet Radio Service) is a standard that adds general Internet Protocol (IP) data communication, such as high-speed Internet and email data services, to GSM networks. GPRS uses a packet-mode technique to transfer high-speed and low-speed data and signaling in an efficient manner over GSM radio networks. The packet radio principle of GPRS can be used for carrying end user's packet data protocol (such as IP and X.25) information from/to a GPRS terminals to/from other GPRS terminals and/or external packet data networks.
GPRS is defined by various ETSI (European Telecommunications Standards Institute) specifications such as ETSI GSM 05.08 version 6.5.0 Release 1997, “Digital cellular telecommunications system (Phase 2+); Radio subsystem link control”; and ETSI GSM 04.60 version 6.4.0 Release 1997, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station (MS)—Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol.” See <http://www.3gpp.org>.
GPRS uses a time division multiple access (TDMA) scheme. In a TDMA scheme, over a given RF channel, each mobile station in a cell transmits and receives (to and from the base station) audio data and non-audio data packets during dedicated time slices or time slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiple access (CDMA), and combinations of such schemes.
In GPRS networks, the allocation of GPRS radio channels is flexible: from 1 to 8 radio interface timeslots can be allocated per TDMA frame. Timeslots are shared by the active users, and uplink and downlink timeslots are allocated separately. The downlink refers to transmissions from the BTS to one or more mobile stations, while uplink refers to transmissions received by the BTS. The radio interface resources can be shared dynamically between speech and data services as a function of service load and operator preference.
In GPRS networks, timeslots are further subdivided into blocks. For example, one block of data is transmitted by a base station on a timeslot every 20 ms. These data blocks, sometimes referred to as Radio Link Control/Medium Access Control (RLC/MAC) blocks, contain a number of bits of data (e.g., 456 physical layer bits). A block can be intended for a particular mobile station. In addition, each downlink block contains a header containing control information called an Uplink State Flag (USF) that must be decoded by all mobiles in the cell of the BTS which are sending data uplink. The USF is used to coordinate uplink transmissions.
To exploit a wide range of carrier-to-interference (C/I) ratios available at different locations within a cell, GPRS networks employ four different airlink coding schemes. GPRS's lowest rate code, CS-1, employs a relatively high number of redundancy bits and offers a maximum LLC-layer throughput of 8 kbps/timeslot. The high level of redundancy present in blocks encoded using CS-1 ensures that mobile stations at the fringes of a cell, where C/I levels are typically lowest, are able to send and receive data. In contrast, the “highest rate” code, CS-4, offers maximum LLC-layer throughputs of 20 kbps/timeslot. Because of the small number of redundancy bits added to each block encoded with CS-4, however, airlink errors can be detected, but not corrected. As a result, CS-4 offers the best airlink performance at relatively high C/I ratios. The remaining two GPRS coding schemes offer maximum LLC-layer throughputs of 12 kbps/timeslot (CS-2) and 14.4 kbps/timeslot (CS-3).
By monitoring the quality of the airlink, mobile stations and the GPRS network can select the coding scheme that offers the best performance. The process of dynamically selecting the coding scheme based on airlink quality is called link adaptation. Quality can be measured, for example, by measuring the bit error rate (BER) or block error rate (BLER) of the channel.
GPRS networks have a single degree of freedom for selecting the coding used to transmit blocks over the air interface, the data encoding scheme to use for each radio block. In contrast, Enhanced GPRS networks (EGPRS) enjoy two degrees of freedom: selection of a modulation scheme—Gaussian Minimum Shift Keying (GMSK) or 8-ary Phase Shift Keying (8-PSK)—and selection of a data encoding scheme. A total of nine different Modulation and Coding Schemes (MCSs) are defined by the EGPRS system specifications.
Table 1 provides a summary of the maximum LLC-layer throughputs per timeslot achieved by each of EGPRS's modulation and coding schemes:
TABLE 1ModulationMaximum LLC-SchemeFamilySchemelayer throughputMCS-1CGMSK8.8kbps/timeslotMCS-2BGMSK11.2kbps/timeslotMCS-3AGMSK14.8kbps/timeslotMCS-4CGMSK17.6kbps/timeslotMCS-5B8-PSK22.4kbps/timeslotMCS-6A8-PSK29.6kbps/timeslotMCS-7B8-PSK44.8kbps/timeslotMCS-8A8-PSK54.4kbps/timeslotMCS-9A8-PSK59.2kbps/timeslot
Four GMSK-based modulation and coding schemes used by EGPRS (MCS-1 through MCS-4) are defined. These coding schemes also provide backward compatibility with GPRS networks. The data encoding schemes used to send downlink blocks encoded with MCS-1 through MCS-4, for example, are encoded in such a way that GPRS mobiles are also able to decode the USF embedded in each downlink block. Hence, GPRS and EGPRS mobiles can have active Temporary Block Flows (TBFs) on the same timeslot simultaneously. A TBF is a radio link connection between a mobile station and a BTS of a network.
Additionally, EGPRS networks have introduced an RLC/MAC re-segmentation scheme as another tool to enable seamless switching between coding schemes in time-varying channels. The scheme allows errored RLC/MAC blocks to be re-segmented into an integral number of RLC/MAC blocks that are re-transmitted using a stronger modulation and coding scheme. To implement the scheme, EGPRS's nine modulation and coding schemes are divided into three coding families. Re-segmentation can only be done using MCSs belonging to the same family.
Due to its higher-level modulation, under similar channel conditions, 8-PSK modulation is not as robust as GMSK. To overcome the loss in performance introduced by 8-PSK modulation and to help increase airlink capacity, EGPRS networks have introduced another substantial improvement to the radio link control layer: support of incremental redundancy.
In systems employing incremental redundancy, re-transmissions of errored blocks carry additional redundancy bits to help the receiver correctly decode the block. In this manner, additional redundancy is added only when needed, potentially increasing the throughput of the airlink. Fields in the header identify the sequence number of the block and the redundancy scheme applied by the transmitter. The receiver can jointly decode multiple versions of the same block—so-called soft combining—improving receiver performance.
A link adaptation algorithm must select the proper modulation and coding scheme to use on the airlink. It must also determine whether blocks should be re-segmented before retransmission. The algorithm must adapt to changes in airlink quality.
Dynamic power control is a tool used for mitigating co-channel interference and extending mobile battery life in wireless networks. In GPRS/EGPRS networks, keeping co-channel interference levels low may enable high rate coding schemes to be used over the airlink. The lower interference levels achieved by power control can result in higher airlink throughputs over larger portions of the cell containing the BTS and mobiles, potentially increasing a cell's data traffic carrying capacity. Effective power control also ensures that timeslots used for EGPRS/GPRS applications do not cause unacceptable levels of interference to timeslots used for voice calls in co-channel neighbor cells.
Circuit-switched (e.g., voice) GSM and EGPRS networks use airlink resources in dramatically different ways. Circuit-switched GSM mobile stations have dedicated use of a single timeslot (or half a timeslot, for half rate users) for the entire duration of a call-periods of time that typically last tens of seconds to minutes. Additionally, these circuit-switched GSM mobile stations report channel quality measurements roughly twice a second.
In contrast, EGPRS users share a single timeslot simultaneously with several EGPRS or GPRS users in a cell. A single user may transmit and receive EGPRS data over multiple timeslots simultaneously. EGPRS radio link connections, i.e., TBFs, can be short-lived, lasting less than a few hundred milliseconds. The BTS measures the quality of each received uplink block from a mobile station. Since the EGPRS network polls mobiles with active uplink TBFs at will, airlink quality measurement reports sent by EGPRS users tend to be more sporadic than in circuit-switched GSM networks. In addition, messages carrying power control commands may not be sent at regular intervals.
In general, the GPRS standards specify various downlink power level requirements that specify, for example, a maximum permitted power level, a minimum permitted power level, and intermediate power levels. For example, a given GPRS network may specify that a downlink block must be transmitted at a power level no greater than a maximum wattage, and at a power level no lower than 10 dB lower than the maximum, and 2 dB increments between. The transmission has to be strong enough to be received by the intended mobile station, with acceptable quality (BER). Moreover, all blocks having header information intended for broadcast to all mobile stations in the cell must be transmitted at a strong enough transmit power to be received by all mobile stations of the cell, even those at the remote fringe or border of the cell.
It is desirable to reduce the downlink power as much as possible, while achieving acceptable airlink quality, so as to reduce co-channel interference. In conventional GPRS systems, a group of blocks, e.g. 10 blocks, are transmitted at a given power level. The base station typically polls a given mobile station having an active link at the end of the group of blocks (i.e., every measurement or reporting interval), to determine the airlink quality. The power level for the next group of blocks can then be adjusted, based on the BER measured at the prior power level, in an attempt to reduce power as much as possible while achieving an acceptable BER.
Uplink power control mechanisms in wireless packet data networks such as GPRS/EGPRS networks may allow the network to tune the transmit power used by each mobile station transmitting uplink RLC/MAC blocks. Uplink power control should provide an important added benefit: transmit power used by each mobile station can be reduced to levels adequate to achieve proper airlink performance, and no higher. Transmit power can be kept as low as possible without sacrificing airlink throughput, giving users peak airlink performance without unnecessarily draining the mobile station's battery.